Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen use as aircraft starter batteries and in aerospace applications, because they are rechargeable, have an extremely long cycle life and provide a uniform output during the entire discharge cycle.
In the typical nickel oxide-hydrogen battery, the battery cells are sealed in an outer pressure vessel that contains pressurized hydrogen gas. On discharge of the battery, the hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates or electrodes and becomes disassociated to the mono-atomic form. The mono-atomic hydrogen is ionized and combines with hydroxyl ions to form water with an electron being released in the process of forming each hydrogen ion. In addition, hydroxyl ions are formed at the positive electrode by the reaction of water with the available oxygen content of the nickel-oxide. As a result of these reactions, an electron current is produced in an exterior circuit.
On recharging, the reaction is reversed, with the recharging being characterized by the regeneration of the hydrogen at the negative electrode and the reoxidation of the nickel-hydroxide at the positive electrode.
In use, a typical metal oxide-hydrogen battery may be subjected to thousands of discharging and charging cycles, and it is recognized that the operating hydrogen pressure range increases as the battery is cycled. For example, at a fully charged state, the hydrogen gas pressure in an individual pressure vessel (IPV) may originally be in the neighborhood of about 600 psi. After several thousand cycles, the hydrogen pressure at the fully charged state, may increase to a value of about 900 psi. In a common pressure vessel (CPV) containing a number of cells, the hydrogen pressure may originally be about 300 psi at the fully charged state and after cycling can increase by 0.03 to 0.30 psi per cycle, depending upon rate and temperature.
It is desirable to utilize the hydrogen pressure within the vessel as a means for indicating the state of charge of the battery, or for charge and discharge control. However, if the pressure at any given state of charge substantially increases with cycling, the pressure cannot be satisfactorily used for these functions. For example, it would be desirable to use the hydrogen pressure to determine the state of charge of the battery and prevent overcharge. However, if the operating hydrogen pressure range increases through cycling, a pressure measurement which would normally indicate full charge would actually be an indication of less than full charge, due to the increase in hydrogen pressure, thus providing a false indication as to the state of charge of the battery.